The prior art in high-average power microwave window technology is the ceramic output window. Such windows are typically constructed from low loss ceramic materials such as alumina (Al.sub.2 O.sub.3) or beryllia (BeO). The disadvantage of alumina is its low thermal conductivity, which limits its power handling capacity. While BeO has a much higher thermal conductivity than alumina (196 W/m.multidot..degree. C. at 100.degree. C.), it is highly toxic in powder form and its use is being discontinued by the microwave tube industry. Windows of all-diamond construction are just now coming into use in high frequency tubes (X- and Ku-band TWTs and millimeter-wave gyrotrons, for example); for such frequencies, however, the windows are relatively small and the cost of the window is a small fraction of that of a high-dollar value microwave tube whose cost can easily exceed $200 K. At frequencies closer to 1 GHz, the size of the window makes the cost of an all-diamond window prohibitive.
Prior art approaches typically involve cooling fins. For example, U.S. Pat. No. 5,051,715, issued to G. Agosti et al, discloses a coupling-out window for linearly polarized microwaves. The coupling-out window comprises, for example, three cooling fins, a plate, which is transparent to microwaves, with strip-like portions as well as an annular mounting. The cooling fins are situated together with the plate in a common plate plane and are, according to a preferred embodiment, of the same thickness as the plate, so that the two plane main surfaces of the plate are formed. The cooling fins are aligned perpendicular to a direction of polarization of the microwaves and are in heat-conducting and pressure-locking contact with the plate.
U.S. Pat. No. 4,458,223, issued to W. Schmidt, discloses a microwave window assembly having cooling means. The microwave window assembly has a ceramic window, such as alumina or beryllia, with a thickness of more than 10 mm, corresponding to a half wavelength of the microwave energy. The window has a metallized side surface, e.g., copper, and is connected by means of a soldered joint to a frame.
U.S. Pat. No. 5,627,642, issued to D. G. Paquette, discloses a method of making a radar transparent window material operable above 2,000.degree. C. and possessing high tensile strength. The method comprises blending a powder mixture of 20 to 60 wt % silicon nitride, 12 to 40 wt % boron nitride, 15 to 40 wt % silica, and 1 to 20 wt % oxygen-carrying sintering aids. The mixture is molded to shape as a preform and is densified by the simultaneous application of pressure and heat to form a monolithic window. The resulting radar transparent window is characterized by a monolithic microstructure consisting of Si.sub.2 ON.sub.2, suspended BN particles, silicon nitrides, various oxynitrides, and silicate materials associated with the oxide sintering aids and minimal unreacted silicon.
U.S. Pat. No. 5,400,004, issued to C. P. Moeller, discloses a distributed window for large diameter waveguides. The window comprises a stack of alternating dielectric and hollow metal strips, brazed together to form a vacuum barrier. The strips are oriented to be perpendicular to the transverse electric field of the incident microwave power. A suitable coolant flows through the metallic strips. The metallic strips are tapered on both sides of the vacuum barrier, which taper serves to funnel the incident microwave power through the dielectric strips.
U.S. Pat. No. 4,536,442, issued to H. P. Bovenkerk et al., discloses a process for making diamond and cubic boron nitride compacts for optical windows. In making the single layer diamond windows, utilization of relatively large diamonds is preferred. The single layer will result in straight-through light paths and the catalyst/binder phase in the matrix would not interfere with transmittance. The compact windows are made by exposing a sample of diamond, for example, in a diamond and graphite matrix, to high pressure-high temperature conditions.
The foregoing references either require complex structures to remove heat from the microwave window or fail to address the problem of enhancing thermal conductivity of the window to enable it to transmit higher average RF power levels. Thus, what is needed is a high-power microwave window having an enhanced thermal conductivity while avoiding most, if not all, of the problems of the prior art.